In recent years, surface-treated steel sheets produced by rustproofing steel sheet materials, particularly, excellently rustproof hot-dip galvanized steel sheets and galvannealed steel sheets, have been used in the fields of automobiles, household electrical appliances, and construction materials.
In general, hot-dip galvanized steel sheets are produced by the following method. First, a slab is subjected to hot rolling, cold rolling, and heat treatment to form a thin steel sheet. The surface of the steel sheet is washed by means of degreasing and/or pickling in a pretreatment step. Alternatively, without the pretreatment step, oils on the surface of the steel sheet are burned in a preheating furnace. The steel sheet is then heated in a nonoxidizing or reducing atmosphere for recrystallization annealing. The steel sheet is then cooled in a nonoxidizing or reducing atmosphere to a temperature suitable for coating and is immersed in a hot-dip galvanizing bath without exposed to the air. The hot-dip galvanizing bath contains a minute amount of Al (approximately 0.1% to 0.2% by mass). Thus, the surface of the steel sheet is coated and becomes a hot-dip galvanized steel sheet. Galvannealed steel sheets are produced by heat-treating hot-dip galvanized steel sheets in an alloying furnace.
In recent years, in the automotive field, steel sheet materials have had higher performance and reduced weight. Increasing strength of steel sheets in order to compensate for strength reduction resulting from weight reduction of steel sheet materials is realized by the addition of solid-solution strengthening elements, such as Si and Mn. In particular, Si can advantageously increasing strength of steel without decreasing ductility. Thus, Si-containing steel sheets are promising high-strength steel sheets. However, the following problems occur in the production of hot-dip galvanized steel sheets and galvannealed steel sheets, when high-strength steel sheets containing large amounts of Si are used as base material.
As described above, hot-dip galvanized steel sheets are annealed in a reducing atmosphere before coating. However, because of its high affinity for oxygen, Si in steel is selectively oxidized even in a reducing atmosphere and forms oxides on the surface of steel sheets. These oxides decrease the wettability of the surface of the steel sheets and form uncoated areas in a coating process. Even when uncoated areas are not formed, these oxides decrease the adhesiveness of the coating.
Several techniques are disclosed in order to address these problems. Patent Literature 1 discloses a technique for improving the wettability of a steel sheet by molten zinc by forming iron oxide on the surface of the steel sheet in an oxidizing atmosphere and then forming a reduced iron layer on the surface of the steel sheet by reduction annealing.
Patent Literature 2 discloses a technique for ensuring high coating quality by controlling the atmosphere, such as the oxygen concentration, in a preheating operation.
Patent Literature 3 discloses a technique of producing a hot-dip galvanized steel sheet that has no uncoated area and has good appearance by dividing the heating zone into three zones A to C and appropriately controlling the temperature and oxygen concentration of each of the heating zones to reduce the occurrence of indentation flaws.